Question

A 1-form "phai" is zero at a point p provided phai(Vp)=0 for all tangent vectors at p. a point at which its diffrential df is zero is called a critical point of the function f.prove that p is a critical point of f if and only if
df/dx(p)=df/dy(p)=df/dz(p)=0 d=dava
find all critical point of f=(1-x2)y+(1-y2)z